Modulation system



1942- F. E. IER MAN MODULATION SYSTEM Filed Jan. 9, 1940 1 INVENTORFAD1?/6'/f5 ERMA/Y 5 5 BY 7 ATTORNEY 2 Patented Jan. 20, 1942 MODULATIONSYSTEM Frederick E. Terman, Stanford University, Caiif., assignor toInternational Standard Electric Corporation, New York, N. Y., acorporation of Delaware Application January 9, 1940, Serial No. 313,035

19 Claims. (01. 179-1715) The present invention relates to systems fordelivering modulated carrier waves, and especially to such systemswherein the wave is delivered by a vacuum tube whose grid is energized,preferably with modulating signal and unmodulated carrier, from twodriver sources of poor regulation, the biases on such vacuum tube beingso related to the amplitude of excitation of the grid that gridcurrentis drawn periodically during intervals corresponding to maximumamplitude of the output wave.

It is an object of the present invention to provide such a systemwherein the distortions which tend to result from the drawing of gridcurrent modulation arrangement wherein the modulating tube is separatelyfed with unmodulated carrier frequency and audio frequency energy fromseparate driving sources at least one of which has inherently acomparatively poor voltage regulation. In such systems ordinarily forthe sake of obtaining reasonably high outputs from the modulating tubethe grid of this tube is so biased that it becomes conductive during thepositive peaks of the impressed audio voltage, and this conductivity ofthe grid, together with the poor regulation of the carrier and/or audiodriver, distorts the voltage delivered by such carrier and/or audiodriver, thus giving a corresponding distortion in the envelope of themodulated output wave. In known systems of this type such distortion hasbeen avoided only by designing the' carrier and audio driver sources soas to have inherently good voltage regulationm inherently expensiverequirement-or by designing these driver sources to deliver much morepower than is actually required even in the intervals of maximum gridcurrent, and then dissipating the greater part of this power in aswamping resistor so that the effective load on these driver sourceswill not vary greatly when Then absorption means are provided fornormally absorbing a part of this power when the grid is non-conductive,these absorption means being varied automatically in response to theflow of the grid current itself, so as to decrease the absorptionthereofat those moments when the grid itself is conductive and thus requiresthe whole power output of the drivers.

The invention may be best understood by reference .to the attacheddrawing in which:

Figs. 1 and 2 represent two embodiments of the invention respectivelyemploying series and parallel connections of the driver sources;

Figs. 3, 4 and 5 represent circuit modifications which may be made inthey circuits of Figs. 1 and 2 in accordance with alternative forms ofmy invention;

Figs. 6, '7, 8 and 9 represent still further circuit modifications whichmay be made in the circuits of Figs. 1 and 2 so as to secure stillfurther, benefits in accordance with a further I feature of myinvention.

Referring more particularly to Fig. 1, this circuit, with the exceptionof the portions shown in heavy lines, represents a conventional type ofmodulator comprising a modulator tube I whose grid is excited from audiosource 2 and carrier source 3 via audio transformer l and tuned radiofrequency coupling circuit 5, respectively. The audio transformer 4 isby-passed by a condenser i which ofiers negligible impedance to carrierfrequencies but a high impedance to audio frequencies. The radiofrequency coupling circuit 5 comprises primary coil! loosely essentiallyconventional, comprising an output tank circuit which consists of coil Htuned by variable frequency condenser l2 and fed at an intermediatepoint with positive potential from the plate supply source l3.The'neutralizing condenser II is connected between the grid and the endof the tank circuit remote from the anode, to provide neutralization inwell known manner.

In accordance with my invention the additiona1 apparatus shown in heavylines is added to the conventional type of modulator above described,for the purpose of improving the effective voltage regulation of carriersource 3, thus enabling the latter to be designed with smaller capacityand' poorer regulation than in conventional systems. This additionalapparatus comprises a rectifier I5 (which is preferably of the spacedischarge type) connected in series with a battery I6, this whole seriescombination being then shunted across the carrier input a shown. Thevoltage of battery Iiis preferably selected to be equal to the desiredpeak amplitude of the carrier input during conditions of maximumconductivity of the grid of tube I. In other words, the voltage ofbattery is preferably made equal to the amplitude of -e carrier inputvoltage at those moments when the voltage from audio source 2 has itsmaximum positive value so that the grid of modulator Inbe I is drawingthe greatestgrid current. Such value may be computed from the knowncharacteristics of the carrier source and the known conductivity of themodulator grid lmder peak voltage conditions. Alternatively this valuemay readily be measured by replacing transformer 4 with a direct currentvoltage equal to the peak audio voltage to be handled and then directlymeasuring the amplitude of the carrier wave across the input 8-9.

when the battery I6 is so adjusted the rectifier I5 will draw no currentduring peak audio intervals. During other portions of each audio cycle,however, when the grid of tube I is nonconductive, (or at any rate lessconductive than during the peak intervals above discussed), theelimination or reduction of grid current will tend to cause the voltageacross the input 8-4 to rise because of the poor regulation of source Assoon as such a voltage rise starts to occur the rectifier I5 will.commence to draw current, thus absorbing energy from source 3.Preferably the impedance of rectifier I5 under the conditions of zero(or minimum) grid current in tube I should I be comparatively low withrespect to the rest of the circuite. g., less than one-sixth of theimpedance of input 3-9, and preferably less than one-thirtieth thereof.Thus even a slight voltage rise will result in a relatively highdissipation of energy inlthis rectifier I5. Such dissipation of energywill therefore limit the voltage rise to a comparatively small value,thus reducing the distortion of a small or preferably negligiblepercentage.

It will be seen therefore that even with the very simple form ofinvention shown in Fig. 1, the voltage variations produced by thevarying conductivity of 'the grid of output I are substantiallycompensated by varying the power absorption of the circuit branch I5, Iiin such manner that the sum of the absorptions of this circuit branchand of the grid of tube I is substantially constant.

It should be noted that the variations of the dissipation of the branchI5, I6 are not merely produced in synchronism with the variations ofconductivity of grid I (as might be the case if V the dissipation ofthis branch were controlled from the audio source 2), but, rather arecon trolled by the grid current itself, thus being dependent upon thevary factor which is to be compensated. This is an important featureoithe present invention since it insures that no matter what factors maychange the grid conductivity, the compensating change in dissipation ofthe absorption branch I5, It will always automatically adiust itself tocorrespond with the grid dissipation of hibe I. Furthermore, the curvewill accurately simulate the complex curve of variation of griddissipation in tube I, thus giving a more perfect compensation thancould be readily obtained by a regulating arrangement which wascontrolled by some other factor than the grid current itself.

Fig. 2 represents another embodiment essentially similar to that of Fig.1, except that the basic modulating circuit is of the well-known shuntinput type rather than of the series input yp shown in Fig. 1. Thedifference is essentially that audio transformer 4 is connected betweenthe grid and cathode of modulator tube I in shunt to "the carrier input8-9 rather than in series therewith. Preferably, a radio frequency chokeI1 is added in series with this audio transformer 4 so that the carrierfrequency cannot back up therethrough. Since this shunt type ofmodulator circuit is in itself well-known, no further explanationthereof is believed necessary. Because of the close similarity betweenthis circuit and that of Fig. 1 corresponding parts are correspondinglynumbered in both figures.

The operation of the absorption circuit I5, I8 is the same in thecircuit of Fig. 2 as in that of Fig. 1, the voltage of battery I6 beingpreferably adjusted so that this circuit draws substantially no currentduring audio peaks and the impedance of rectifier I5 being preferably solow that a very S g t rise in radio frequency voltage causes the circuitI5, I8 to dissipate considerable energy, thus limiting such voltage riseto a negligible amount.

Figs. 3-9 represent circuit modifications which may be substitutedeither in the circuit of Fig. l or that of Fig. 2, the fragmentarycircuits illustrated in Figs. 3-9 being intended to directly replacethose portions of Figs. 1 and 2 which are enclosed within the dot-dashlines.

Referring to Fig. 3, this differs from the arrangement of Figs. 1 and 2in that battery it is replaced by the combination of a cathode resistorI63 and a by-pass condenser ItA. Preferably, this condenser 56A is solarge as to presentnegligihle impedance not only with respect to thecarrier but also with respect to audio frequencies, so

that the potential across this condenser will be substantially constantover a period covering several cycles of the lowest desired audiofrequency. For convenience in understanding the invention directly fromthe drawing this condenser I5A has correspondingly been illustrated ashaving several plates in order to indicate in a rough way that itscapacity is higher than that of an ordinary by-pass condenser. Similarlyin the other figures of drawing all condensers intended to havenegligible impedance with respect to both audio and radio frequenciesare represented in similar manner.

The modified circuit of Fig. 3 operates essentially like that of Figs; 1and 2. It is true that this condenser resistor circuit IGA and ISB willregulate the voltage of the carrier input to a very slightly lowervoltage during the transmission of modulated waves than during thetransmission of unmodulated carrier. This is because during each audiopeak the rectifier I5 will momentarily cease to pass current andtherefore the mean current averaged over several audio cycles will tendto be slightly less when modulation is present than when it is absent.The voltage of the resistor-condenser combination I ISA-I53 willtherefore fall off. This falling oi! of voltage, howof variation of thedissipation of branch 15, it ever, will be extremely small, because theslightest reduction 01' this voltage immediately cause a sharp increaseof the current through rectifier IS, thus quickly checking the voltagereduction. The only effect of this will be that the carrier amplitudewill rise very slightly in the absence of modulation; which isordinarily of no consequence.

Fig. 4 represents another modification of Figs. 1 and 2 which permitsthe absorption circuit IS, IS to be connected to ground, thus renderingit more convenient to excite the cathode thereof and in other waysfacilitating the practical design. In the arrangement of Fig. 4 theabsorption circuit IS, I8 is not galvanically connected across thesecondary 8-8 of the carrier coupling arrangement but is effectivelyconnected thereacross by connecting this absorption circuit to an extrawinding I8 which is closely coupled inductively to the winding 8. Thetransformer thus constituted by windings 8 and I8 may, if desired,

have a voltage step-up so that the impedance of rectifier IS as viewedfrom coil 8 will appear to be even lower than it actually is, thusenabling a still closer voltage regulation to be obtained.

The circuit modification shown in Fig. 5 is very similar to that of Fig.4, excepting that a triode ISA replaces the diode IS. correspondingly,the battery I6 is connected between grid and cathode of the triode ISAratherthan between anode and cathode thereof, thus permitting the use ofa lower voltage battery and enabling the cathode of tube ISA to bedirectly grounded.

It should be clear, however, that the position of the battery in this orany other figures may be altered in many ways without essentiallychanging the operation of the invention. For example, battery IS in Fig.4 could be connected between coil I8 and the grounding point, thuspermitting the 'direct grounding of the cathode of tube IS; or the sameeffect could be produced by connecting the battery I6 between anode andwinding I8inFig. 4. Similarly, in Fig. 5 battery It can be connected inthe grid lead as shown, in the cathode lead, in the anode lead, or inthe lead between coil I8 and ground. Also in the later figures hereafterdescribed the positions of the batteries can be similarly changed withgood results. In Fig. 6, for example, battery I6 may be connected nextto the cathode to bias both the grid and anode with some advantages. Ifdesired a plurality of batteries can be used simultaneously in severalsuch positions.

In the last four Figures 6, 7, 8 and 9, triode ISA is used as in Fig. 5and is inductively coupled to the carrier input as in Figs. 4 and 5.These last four figures essentially differ from those previouslydescribed, however, in that each of them includes a leak-and-condensercombination I9-2II connected in series with the grid of modulator tube Iso as to be traversed by the grid current'thereof, and connections forcontrolling the conductivity of the triode ISA in accordance with thevoltage drop developed across such leak-andcondenser combination Ill-20.By such an arrangement it is possible to fully compensate for thevoltage drop produced by increasing conductivity of the modulator gridor even ii desired to produce overcompensation so that the carrieramplitude actually rises at the times of maximum grid current.

Referring more particularly to Fig. 6' the absorption triode ISA has itsanode-cathode path tightly coupled to the carrier input circuit 8-9 byextra coil I8 as described inconnection with Figs. 4 and 5. The grid ofthis absorption triode ISA is biased by battery I8 preferably to such anegative value that the tube I SA will not pass any substantial currenteven at the radio frequency peaks, so long as the maximum grid currentis flowing in the grid of modulator I and is thus producing a voltagedrop across resistor I8 to form an additional negative bias for the gridof the absorption triode ISA. If the circuit is so adjusted, then whenthe grid of the modulator tube is not drawing its minimum grid current(i. e. during the valleys or intermediate portions of the audiofrequency cycle), the absorption triode ISA will draw a pulse of radiofrequency current at each radio frequencypeak, thus absorbing power fromcarrier source 3.

By proper adjustment of the grid leak IS the amount of power dissipatedin triode ISA during the intervals when the main grid current throughresistor I9 is zero can be made equal to, greater than, or less -thanthe amount of power dissipated by the grid of the modulator tube. I whenthe latter draws grid current at the positive peaks of an audio cycle ofthe greatest amplitude to be handled. Correspondingly, therefore, thecircuit arrangement of Fig. 6 will act to exactly offset,over-compensate or under-compensate for the changes in carrier voltagewhich result from changes of grid current in the modulator tube.

It will be noted that even with a zero value of leak resistor I9 anapproximate voltage regulation will be obtained especially if tube ISAhas a low impedance compared to input ISA because the circuit will thenresemble that of Fig. 5 and will absorb increasing amounts of power inresponse to any rise in radio frequency voltage which may result fromthe decrease of modulator tube grid current as the applied audiofrequency voltage drops away from its maximum peak value. With a smallvalue of resistor I8, not large enough to give full compensation, thecircuit will therefore operate partially by the control effect of thisresistor and partially according to theprinciples of Fig. 5. 0n theother hand, if resistor I9 is a little too large so as to tend toproduceovercompensation, the amount of overcompensation will be veryslight; because the rise in radio frequency voltage will be limited dueto the increased conductivity of tube ISA which results from theincreased voltage across its anode and cathode. Thus it will be seenthat the value of resistor I9 is not at all critical, at least when tubeISA is of low impedance. This avoids any possible need for readjustingthis resistor when tubes are changed.

In accordance with a further feature of my invention, a circuitarrangement such as shown in Fig. 6, is adjusted so as to not onlycompensate for the changes in carrier frequency voltage which resultfrom varying conductivity of the modulator grid but also to considerableovercompensate for these changes so that the carrier frequency voltageapplied to the modulator grid rises substantially at those times whenthe modulator grid draws the most grid current. For this purpose theanode cathode circuit is preferably given. an impedance not less thanthat of the input 8-9, or at any rate not less than a third this value.By means of such overcompensation it is possible to correct anydistortions introduced by the falling off of the audio frequency voltagewhich also occurs at times of maximum modulator grid conductivity if theaudio driver 2 has imperfect regulation. I

' as a cathode self-bias for the amplifier 2|.

When so adjusted the circuit of Fig. 6 does not prevent the audiofrequency voltage from falling off at times of maximum modulator gridconductivity, and thus does not prevent a slight flattening of the peaksof the input audio frequency voltage applied to the modulator grid. Thecircuit of Fig. 6 can, however, be made to compensate for thedistortions in the modulated wave output of the modulator tube byadjusting this circuit to produce sufficient overcompensation of theradio frequency voltages. When this is done the total effect will bethat at moments of maximum grid current in the grid of the modulatortube I the audio frequency voltage will slightly fall off, butsimultaneously and correspondingly the radio frequency voltage willrise. If these two opposite effects are properly proportioned withrespect to the characteristic of the modulator tube itself, the netresult can be made to be the same as if both the radio frequency andaudio frequency driver had such perfect regulation that neither of thesevoltages were changed in any manner when the grid of the modulator tubebegan to draw'grid current.

Fig. 7 represents a modification of Fig, 6 wherein the battery I6 isreplaced bya resistorcondenser combination |6A, IBB, somewhat as in thearrangement of Fig. 3 previously described. Such modification introducessubstantially no change in the manner of operation excepting that themean carrier level of the output wave will very slightly rise, asexplained in connection with Fig. 3, during extended periods of nomodulation or weak modulation, such as in intervals between syllables inthe case of speech or during silent periods on the program. Aspreviously mentioned, this is of little consequence.

Fig. 8 represents a circuit modification wherein an additional amplifiertube is connected between the leak-and-condenser combination I 9,

ing galvanically coupled to the grid of absorption triode ISA. Thecircuit of Fig. 8 like that of Figs. 6 and 7 should preferably beadjusted so that the absorption triode absorbs substantially no powerfrom the carrier source when the modulator grid is drawing its maximumgrid current. In the case of Fig. 8 this adjustment may be made byvarying either the potential of battery 23 or the value of resistor I6B.Preferably both of these are adjusted to bring the tubes 2| and |5A toconvenient points on their operating curves. The amount of controlexerted by Fig. 8, like that of Figs. 6 and 7, depends largely upon thevalue of the grid current resistor l9, and may be adjusted to produceeither exact compensation of the radio frequency voltages orovercompensation thereof-in case it is desired to offset some poorregulation of the audio frequency.

Fig. 9 is very closely similar to the circuit shown in Fig. 8 exceptthat the galvanic connections to tube 2| are replaced by capacitycoupling through the large condensers 24 and 25 so that this amplifier2| merely amplifies the changes in the drop across resistor l9, ratherthan the absolute value of this drop. In addition, the circuit of. Fig.9 dispenses with battery 23 by returning the grid of tube 2| to anintermediate point on resistor 22 so that the grid bias of tube 2| isdetermined only by the upper portion of this resistor 22, whereas thewhole of resistor 22 serves as the coupling resistor for coupling thistube to the triode ISA. This return of the grid of tube 2| to anintermediate point of resistor 22 is effected through a conventionalgrid coupling resistor 26 with which resistor 21 and condenser 28 areassociated so that these three elements form a filter. Such filter 2'I28serves to minimize degenerative 20 and the absorption triode |5A for thepurpose of amplifying the voltage drop produced by suchleak-and-condenser combination to provide a greater range of variationof the impedance of the triode I5A. This figure is, in many respects,similar to Fig. "l and corresponding parts are correspondingly numberedto facilitate comparison thereof.

In the circuit of Fig. 8 an additional amplifying tube 2| is providedwhose input circuit is connected across the resistor-and-condensencombination I9, 20 and whose anode cathode circuit is coupled to thegrid of the absorption triode |5A by way of cathode coupling resistor22. This cathode coupling resistor 22 also acts Because of the fact thatthis resistor must also serve as the coupling resistor for the output oftube 22, the impedance thereof is preferably more than sufficient togive suitable bias for tube 2| itself, and accordingly a battery 23 isconnected in series therewith to partially nullify the drop in thisresistor 22 so as to bring the grid bias of tube 2| to a desired value.This battery 23 also serves to render the grid of tube |5A negative withrespect to the corresponding cathode so that in some cases thecondenser-and-resistcr combination ISA, |6B shown in Fig. 8 may beomitted.

It will be noted that tube 2| serves as a direct current amplifier, itsinput being galvanically coupled to the resistor |9 which develops thecontrol voltage (in response to grid current through the modulatortube), and its output becoupling which might otherwise occur as a resultof the common impedance constituted by the lower half of resistance 22.

The circuit of Fig. 9, like those of Figs, 6, 7 and 8, may be made toexactly compensate or overcompensate for radio frequency voltagevariations, thus being capable of taking care of poor regulation notonly in the carrier frequency driver 3 but also in the audio frequencydriver 2, or in the latter alone if the carrier source inherently hasgood regulation. As in the other figures,-grid current resistor I9 maybe used to control the amount of compensation as to adjust the circuitfor exact compensation or overcompensation in any desired degree.

It should be noted that in the various embodiments of Figs. 1-9 theatfiustment of battery I6 (or of the corresponding resistor-condensercombination ISA-I63) is not. critical. Although it is preferred toadjust this battery so that no unnecessary energy absorption will occurin the absorption circuit, nevertheless the arrangement will operatesatisfactorily even if the battery has a somewhat lower voltage. In thelatter case the only result will be that the maximum carrier amplitudewill be automatically reduced to equal the voltage of battery I6, sothat some unnecessary dissipation of power will take place in theabsorption circuit even during the audio peaks.

For convenience, my invention has been described in connection with amodulating system of the type wherein low frequency modulating signalsare employed to modulate a high frequency carrier, thus producing amodulated output wave consisting of a carrier and two sidebands. Theinvention is applicable, however, to all kinds of modulating systemssuch as those delivering suppressed carrier'and/or single sidebandmodulated waves. 'The invention is also applicable to the so-calledheterodyne demodulators or frequency changers wherein a high frequencysignaling wave consisting of one or two side bands is used to modulatethe carrier so as to yield a modulated wave which may be of lowfrequency corresponding 'to the difference between the carrier andsignal waves or of high frequency corresponding to their sum.

In Fig. 1 for example, audio source 2 may be replaced by a source. ofhigh frequency single or double sideband signals, the audio transformer4 being correspondingly replaced by a high frequency transformer, andthe carrier by-pass condenser 6 being omitted or replaced by a seriesresonant circuit of high impedance elements tuned to the frequency ofsource 3 so as to bypass the carrier only. In Fig. 2 similar alterationmay be made in this figure it may also be desirable to omit carrierblocking choke I1 or to replace it by a shunt resonant circuit of lowimpedance elements tuned to block the carrier only. The modificationshown in Figs. 3-9 will still be suitable for application to suchaltered systems using high frequency signal inputs, the condensers ISA,24, and 25 being then made large, not only with respect to the highfrequency signal waves but also with respect to the frequency of themodulated output waves if these are of lower frequency than the inputwaves. Such modified modulation systems may be of the so-called"demodulation or "frequency changing" type, the output wave being eitherhigher or lower than the input signal wave. In fact the frequency rangeof the output wave may even overlap the frequency range of the inputwave if the modulator is balanced in known manner to suppress the inputsignal in the output circuit;

Since all such systems are really modulating systems whether they becalled modulators, demodulators or frequency changers, the expressionsmodulating system, modulating source, modulating voltage, etc., are usedgenerically in the appended claims without regard to whether the inputsignal wave is of low frequency and I the output wave of high frequencyor vice versa.

What I claim is:

l. A modulation system which comprises a discharge tube having acathode, an anode and a grid, means for normally biasing said grid todraw substantially no grid current, a carrier source connected to applyan unmodulated carrier voltage between said grid and cathode, amodulating source connected to apply a further modulating voltagebetween said grid and cathode, the voltages of said sources beingsuflicient to cause the flow of substantial grid current, at leastduring overlapping peaks of said carrier and modulating voltages and thevoltage of at least one .of said sources being subject to substantialvariation with varying load, absorption means charge tube having acathode, an anode and a grid, means for normally biasing said grid todraw substantially no grid current, a carrier source whose voltage issubject to substantial variations with varying load, connected to applyand cathode, a modulating source connected to apply a further modulatingvoltage between said grid and cathode, the voltages of said sourcesbeing sufficient to cause the flow of substantial grid current at leastduring overlapping peaks of said carrier and modulating voltages,absorption means for normally absorbing power from said carrier source,and means responsive to the flow of grid current for decreasing theabsolute power absorbed by said absorption means.

3. A system according to claim 2 wherein said power absorption meanscomprises a circuit incorporating rectifier means and. wherein saidmeans responsive to the flow of grid current for decreasing-the absolutepower absorbed by said absorption means comprises means associatedwithsaid rectifier means for causing the impedance thereof to increasesharply in response to small decreases in said carrier voltage, wherebythe carrier voltage reduction which inherently results from anysubstantial increase in grid current causes a substantial decrease inthe power absorption of said absorption means.

4. A system according to claim 2 wherein said means responsive to theflow of grid current for decreasing the absolute power absorbed by said,absorption means comprises means associated with said absorption meansfor causing the power absorption thereof to decrease sharply in responseto small decreases in said carrier voltage, whereby the carrier voltagereduction which inherently results from any substantial increase in gridcurrent causes a substantial decrease in the power absorption of saidabsorption means.

5. A modulation system which comprises a discharge tube having acathode, an-anode and a grid, means for normally biasing said grid todraw substantially no grid current, a carrier source connected to applya carrier voltage between said grid and cathode, a modulating sourceconnected to applya further modulating voltage between said grid andcathode, the voltages of said sources being suiiicient to cause the flowof substantial grid current at least during overlapping peaks of saidcarrier and modulating voltages, and the voltage of at least one of saidsources being subject to substantial variations with varying load,absorption means for normally absorbing powerfrcm said carrier source,means for deriving from said grid current a voltage dependent on thevalue of said grid current, and means for controlling the impedance ofsaid absorption means from said derived voltage.

6. A system according to claim 5 wherein the voltage of at least saidmodulating source is subject to substantial variations with varying loadand wherein said means forderiving a voltage from said grid current andsaid means for controlling the impedance of saidabsorption means fromsaid derived voltage comprise an impedance connected to be traversed bysaid grid current and proportioned to cause a rise in said carriervoltage in response to increasing grid current.

'7. A system according to claim 2 wherein said means for decreasing saidpower absorption responsive to increasing grid current comprises meansassociated with said absorption means for causing the power absorptionthereof to decrease sharply in response to small decreases in saidcarrier voltage, whereby the carrier voltage reduction which inherentlyresults from any substantial increase in grid current causes asubstantial decrease in the power absorption of said 5 absorption meansand further comprises means [tor deriving from said grid current avoltage dependent on the value of said grid current, and means forcontrolling the impedance of said absorption means from said derivedvoltage.

8. A modulation system which comprises a -a8e is subject to substantialvariation with varying load connected to apply a carrier voltage betweensaid grid and cathode, a modulating source connected to apply a separatemodulating signal voltage between said grid and cathode, the voltages ofsaid sources being suflicient to cause the flow of substantial gridcurrent, at least during overlapping peaks of said carrier andmodulating signal voltages, an output circuit for deriving from saidanode a modulated signal voltage resulting from the modulation of saidcarrier voltage by said modulating signal voltage, a further dischargetube having a control electrode and a space path whose impedance iscontrolled thereb an absorption circuit including said space path andconnected to normally absorb power from said carrier source, andmeansfor increas ing the impedance of said space path sharply responsive tosmall absolute decreases in said carrier voltage.

9. A system according to claim 8, wherein at least one of said signalvoltages is of substantially lower frequency than said carrier voltage,further comprising means for deriving from said grid current a voltagedependent on the mean value of said grid current averaged over a numberof carrier cycles but over only a portion of 'a cycle of said lowerfrequency signal voltage,

and means for applying said derived voltage to said control electrode tofurther vary the impedance of said space path.

10. A system according to claim 8, wherein said modulating signalvoltage is of substantially lower frequency than said carrier voltage,further comprising means for deriving from said grid current a voltagedependent on the mean valueof said grid current averaged over a numberof carrier cycles but over only a portion of a cycle of said modulatingsignal voltage, and means for applying said derived voltage to saidcontrol electrode to further vary the impedance of said space path.

11. A modulation system which comprises a modulating discharge tubehaving a. cathode, an anode and a grid, means for normally biasing saidgrid .to draw substantially no grid current, a

can'ier source connected to apply a carrier voltage between said gridand cathode, a modulating source connected to apply a modulating signalvoltage between said grid and cathode, the voltages of said sourcesbeing suilicient to cause the flow of substantial grid current at leastduring overlapping peaks of said carrier and modulating signal voltagesand the voltage of at least one of said sources being subject tosubstantial variations with varying load, an output circuit forderivingfrom said anode a modulated signal voltage resulting from themodulation of said carrier voltage by said modulating signal voltage, atleast one of said signal voltages being of substantially lower frequencythan said carrier voltage, a further discharge tube having a controlelectrode and a space path whose impedance is controlled thereby, anabsorption circuit including said space path and connected to normallyabsorb power from said carrier source, means for deriving from said gridcurrent a control voltage .dependent on the value of said grid currentsaid means including a resistor and condenser having a time constantcorresponding to anumber of carrier cycles but to only a small part of acycle or said lower frequency si nal voltage, and means for applyingsaidcontrol voltage to said control electrode to vary the impedance ofsaid space path.

12. A modulation system which comprises a modulating discharge tubehaving a cathode, an anode and a grid, means for normally biasing saidgrid to draw substantially no grid current, a carrier source connectedto apply a carrier voltage between said grid and cathode, a modulatingsource connected to apply a modulating voltage of substantially lowerfrequency than said carrier voltage between said grid and cathode, thevoltages of said sources being suiiicient to cause the flow ofsubstantial grid current at least during overlapping peaks of saidcarrier and modulating voltages and the voltage of at least one of saidsources being subject to substantial variations with varying load, afurther discharge tube having a control electrode and a space path whoseimpedance is controlled thereby, an absorption circuit including saidspace path and connected to normally absorb power from said carriersource, means for deriving from said grid current a control voltagedependent on the value of said grid current said means including aresistor and condenser having a time constant corresponding to a numberof carrier cycles but to only a small part of a cycle of said modulatingvoltage, and means for applyi g said control voltage to said controlelectrode to vary th impedance of said space path. r

13. A modulation system which comprises a modulating discharge tubehaving a cathode, an anode and a grid, means for normally biasing saidgrid to draw substantially no grid current, a carrier source connectedto apply a carrier voltag between said grid and cathode, a modulatingsource connected to apply a modulating signal voltage between said gridand cathode to produce by modulation a modulated signal voltage, atleast one of said signal voltages being of substantially lower frequencythan said carrier voltage, the voltages of said sources being suflicientto cause the flow of substantial grid current at least duringoverlapping peaks of said carrier and modulating signal voltages and thevoltage of at least one of said sources being subject to substantialvariations with varying load, a further discharge tube having a controlelectrode and a space path whose impedance is controlled thereby, anabsorption circuit including said space path and connected to normallyabsorb 'power from said carrier source, means for deriving from saidgrid current 'a unidirectional control voltage dependent on the value ofsaid grid current said means including a resistor and condenser having atime constant corresponding to a number of carrier cycles but to only asmall part of a cycle of said lower frequency signal voltage, and meansfor applying said unidirectional control voltage to said controlelectrode to vary the impedance of said space path.

14. A modulation system which comprises a modulating discharge tubehaving a cathode, an anode and a grid, means for normally biasing saidgrid to draw substantially no grid current, a carrier source connectedto apply a carrier voltage between said grid and cathode, a modulatingsource connected to apply a modulating voltage of substantially lowerfrequency than said carrier voltage between said grid and cathode, thevoltages of said source being sufficient to cause the flow ofsubstantial grid current at least during overlapping peaks of saidcarrier and modulating voltages and the voltage of at least one of saidsources being subject to substantial variations with varying load, afurther discharge tube having a control electrode and a space path whoseimpedance is controlled thereby, an absorption circuit including saidspace path and connected to normally absorb power from said carrier Isource, means for deriving from said grid current a unidirectionalcontrol voltage dependent on the value of said grid current said meansincluding a resistor and condenser having a time constant correspondingto a number of carrier cycles but to only a small part of a cycle ofsaid modulating voltage, and means for applying said unidirectionalcontrol voltage to said control electrode to vary the impedance of saidspace path.

15. A modulation system which comprises a first discharge tube having acathode, an anode and a grid, means for normally biasing said grid todraw substantially to grid current, a carrier source connected to applya carrier voltage between said grid and cathode, a modulating sourceconnected to apply a modulating signal voltage between said grid andcathode to produce by' modulating a modulated signal voltage, at leastone of said signal voltages being of substantially lower frequency thansaid carrier voltage, the voltages of said sources being suflicient tocause the flow of substantial grid current at least during overlappingpeaks of said carrier and modulating signal voltages and the voltage ofat least one of said sources being subject to substantial variationswith varying load, a further discharge tube having a control electrodeand a space path whose impedance is controlled thereby, an absorptioncircuit including said space path and connected to normally absorb powerfrom said carrier source, means for, deriving from said grid current analternating control voltage dependent on the value of said grid currentsaid means including a resistor and condenser having a time constantcorresponding to a number of carrier cycles but to only a small part ofa cycle of said to draw substantially no grid current, a carrier sourceconnected to apply a carrier voltage between said grid and cathode, amodulating source connected to apply a modulating voltage ofsubstantially lower frequency than said carrier voltage between saidgrid and cathode, the Volt ages of said sources being sufiicient tocause the flow of substantial grid current at least during overlappingpeaks of said carrier and modulating voltages and the voltage of atleast one of said sources being subject to substantial varia-- tionswith varying load, a further discharge tube having a control electrodeand a spacepath whose impedance is controlled thereby, an absorptioncircuit including said space path and connected to normally absorb powerfrom said carrier source, means for deriving from said grid current analternating control voltage dependent on the value of saidgrid currentsaid means including a resistor and condenser having a time constantcorresponding to a. number of carrier cycles but to only a small part ofa cycle of said modulating voltage, and means for \applying saidalternating control voltage to said control electrode to vary theimpedance of said space source.

drop and wherein said means for deriving a unidirectional voltage fromsaid grid current farther comprises direct current amplifying means foramplifying said unidirectional potential drop to yield saidunidirectional control voltage.

19. A'system according to claim 15 wherein said means for deriving analternating control voltage from said grid current includes an impedancetraversed by said grid current to produce a unidirectional potentialdrop and alternating current amplifying means for amplifying thefluctuations of said unidirectional potential drop to yield saidalternating control voltage. V

FREDERICK E. TERMAN.

